Dual thermoelement system for measuring rapidly changing fluid temperatures and thermo-elements therefor



w. H. GIEDT 3,139,752 LEMENT SYSTEM FOR MEASURING RAPIDLY CHANGING TS THEREFOR July 7, 1964 DUAL THERMOE FLUID TEMPERATURES AND THERMO-ELEMEN 3 Sheets-Sheet l Filed March 16, 1959 INVENTOR. //wefA/ /7, 6m07- July 7, 1964 EDT W. H. Gl DUAL THERMOELEMENT SYSTEM FOR M 3, EASURING RAPIDLY CHANGING EMENTS THEREF'OR 5 Sheets-Sheet 2 FLUID TEMPERATURES AND THERMO-EL Filed March 16, 1959 INVENTOR. /HPPW h. 67507' FIG-7 wlmi;

July 7, 1964 w. H. GIEDT 3,139,752 DUAL THERMOELEMENT SYSTEM FOR MEASURING RAPIDLY CHANGING MENTS THEREFOR 3 Sheets-Sheet 3 FLUID TEMPERATURES AND THERMO-ELE Filed March 16, 1959 INVENTOR. www @for United States Patent O `3,139,752 DUAL THERMOELEMENT SYSTEM FOR MEASUR- ING RAPIDLY CHANGING "FLUID TEMPERA- TURES AND THERMO-ELEMENTS THEREFOR Warren H. Giedt, San Francisco, Calif., assigner to American Radiator & Standard Sanitary Corporation, New York, N.Y., a corporation of Delaware Filed Mar. 16, 1959, Ser. No. 799,719 2 Claims. (Cl. I3- 359) This-inventionrelates tofandin general has for its object the provision of an instantaneously responsive thermoelement system formeasuring a rapidly Varying uid temperature, land to thermoelement probes, such as thermocouple probes, resistance element probes, thermistor probes and vapor-filled bulbs for use in connection with such a system.

The performance of jet and rocket engines is a function of the temperatures lgenerated in the gas streams owing through and leaving the systems. Measurement of these temperatures is essential, not only for the evaluation of engine performance, but also for assuring a safe operating environment for the engine components. The latter, for example, is a necessary consideration in the design of fuel control systems.

'The high velocities, high temperatures, and rapidly changing` conditions in such gas streams make the temperature-measurement problem an extremely difficult and complexone. Consequently, extensive effort has been devotedtothe design and development of equipment for this application. Resultsto date,.however, are admittedly inadequate, ,particularly in regard to response rate under rapidly changing conditions and in the very high temperature regime.

This invention is also applicable in the lield of nuclear reactors. It is possible in `homogeneous reactors (i.e., Where the tissionable material is uniformly dispersed throughout a liquid medium) to cause essentially step changes in temperature by sudden changes in theposition of the control rods. Immediate measurement of the resulting temperature changesis necessary for both experimental study of performance and for control purposes. It should be noted that the fluid in such a case might be quiescent or flowing.

The procedure most commonly employed for determining the temperature of afluid (stationary or iiowing) is to measure the magnitude of sometemperature-dependent property of an object immersed in the fluid. The quantity measured may be the length or volume, the electrical resistance, or generated thermoelectromotive force. The actual specification of temperature involves calibration of the .measured quantity in terms of a specified temperature scale. Inherentin this procedure `is the requirement that the sensing device be in the uid long enough to assume the fluid temperature. When the latter changes, some time will elapse before the sensing element attains the new temperature andprovides the `desired indication.

For `purposes of comparing the `abilities of different elements to (follow rapidly changing uid temperatures, a quantityknown as the time constant Tris defined. -As ordinarily used, this is simply the time requiredfor an element to undergo 63% of an instantaneous (step) change in the `fluid temperature. 7- is clearlydetermined by the thermal capacity of 'the sensing element and the freedom with which heat is transferred to it. Specically, the lower the product of the specific heat and the weight ofthe element, and the higher the product of the area and the coeiicent Aof heat transfer (sometimes called the unit thermal conductance) between the fluid and the element, the smaller 'r will be.

In many instances, however, particularly in the eld of reaction propulsion, even when 1- is made as small as possible (through use of a very small-diameter thermocouple, for example), either huid-stream temperature changes are not sensed rapidly enough, or it is desirable to improve this sensing ability` inorderto improve'system performance.

More specifically, one of the objects of this invention is the provision of a dual thermoelement device for determining the instantaneous `temperature'ofa `iluid (stationary or llowing)` wherein tirst and second thermo-.elements such as thermocouple elements, `resistance elements, thermistors and fluid-filled bulbs, are located in said fluid, such that the total` thermahcon'ductances of said elements are equal, and wherein the thermal capacities of said thermo-elements are unequal.

A second objectof this invention is ltheprovision 4of a dual thermoelement system for determiningthe instantaneous temperature of a fluid wherein first and second thermo-elements such as thermocouple elements, resistance elements, thermistors and duid-filled bulbs are symmetrically located in said iiuid for identical action thereby, wherein the totalthermal conductances of said elements are equal, wherein the thermal capacities of said elements are unequal, `and wherein said elements are connected to a computer capable of instantaneously solving the equation wherein tg=fluid temperature varying in general with time T t1=instantaneous temperature indicated by the .-lirst sensing element tzr-instantaneous temperature indicated Vby the second senseing element W1=weight of the rst sensing element, -and W2=weight of the second sensing element.

fPresent computing machines can be rused to perform the calculations required inthe above equation. Design of a special device for this purpose can also be carried out following established procedures.

AAnother object of `this invention is the `provision of-a system of the characterabove described whereinfthefoutputs of said thermal elements representing their instantaneous temperatures, are transmitted to a `computer capable of `solving the equation t1=instantaneous temperature indicated by the rst sensing element t2=instrantaneous temperature indicated by the second sensing element it the thermal capacity of the first probe W1c1 the thermal capacity of the second probe Wzcg wherein c1=speciiic heat of the first sensing element c2=specicrheat of the second sensing element tg=fluid temperature varying in general with time T t1i=instantaneous temperature indicated by the rst sensing element t2=instantaneous temperature indicated by the second sensing element The invention possesses other Iadvantageous features, some of which with the foregoing, will be set forth at length in the following description where those forms of the invention which have been selected for illustration in the drawings accompanying and forming a part of the present specilication, are outlined in full. In said drawings, several forms of the invention are shown, but it is to be understood that it is not limited to such forms, since the invention as set forth in the claims may be embodied in other forms.

Referring to the drawings:

FIG. 1 is a schematic diagram illustrating a system embodying the objects of our invention and including a computer for solving for tg.

FIG. 2 is a diagram similar to the diagram of FIG. 1,

Vbut wherein each f the thermoelement probes are shown connected through a reference junction with a recording potentiometer in standard fashion rather than being associated with a computer.

FIG. 3 is an enlarged fragmentary perspective view of the first and second thermocouple elements and their ceramic holder included in FIGS. 1 and 2.

FIG. 4 is a view similar to FIG. 3 but wherein resistance elements have been substituted for the thermocouple elements.

FIG. 5 is a view similar to FIG. 4 but wherein the resistance elements are encased in ceramic tubing.

FIG. 6 is another view similar to FIG. 3 but wherein thermistor elements have been substituted for the thermocouple elements.

FIG. 7 is a diagrammatic illustration of an alternative form of a two-thermocouple probe.

FIG. 8 is a wiring diagram for use in conjunction with a resistance type of thermoelement probe.

FIG. 9 is a diagram of a sensing system in which iiuidfilled bulbs are used as thermo-elements instead of thermocouples or resistances.

For a given thermocouple yin a iiuid (stationary or flowing; either of the two shown in FIG. 1), the differential equation describing the thermocouple response is (1) hA(tg-t)=Wct where:

h=heat transfer coefficient (unit thermal conductance) between the fluid and the thermocouple A=exposed area of the thermocouple through which heat transfer occurs tg=uid temperature, which in general varies With time T t=instantaneous temperature of the thermocouple W=weight of the thermocouple c=specif1c heat of the thermocouple t'=dt/ dT :rate of change of the thermocouple temperature Equation l can be rearranged to .(2) gift/Mds, or fff+i=zg where r=T-7==thermocouple time constant it by f, and adds to the product to t to give the instantaneous fluid stream temperature tg. It should be noted, however, that since 1- varies with stream flow conditions (the flow determines h), these systems require adjustment for a change in ow conditions.

The present invention incorporates a dual sensing element and associated equipment compensating for time lag and which is independent of flow conditions.V

As illustrated in FIG. l, such a system includes a conduit 1 serving to cause a fluid stream 2 (the instantaneous temperature tg of which is to be determined) to ilow in a predetermined path. Mounted on the conduit 1 and extending thereinto is a ceramic mounting block 3, and mounted in the block 3 are rst and second thermocouple elements or probes respectively indicated by the reference numerals 4 and 5. Essential to this modification of my invention is the condition that the two thermocouple elements be symmetrically disposed within the path of the fluid stream so that the action of the fluid stream on the two probes will be identical. Although as illustrated Y in FIG. A1, the two probes are located in a plane transverse to the direction of flow of the uid stream, this is not essential, it being merely necessary that the probes be symmetrically located in the fluid stream.

The thermocouple element 4 consists of legs 4a and 4b protruding from the mounting block 3 and joined at their upper ends to form a hot junction 6. The legs 4a and 4b can be made of solid wire of dissimilar metals such as, for example, chromel and Alumel, and of identical diameters. Likewise, the probe 5 includes two legs 5a and 5b made of dissimilar metals such as chromel and Alumel, and joined to form a hot junction 7. A second essential condition of this modification of my invention to be here particularly noted is that although the surface area A of the probe 5 should be identical to the surface area of the probe 4, the thermal capacities of the two probes 4 and 5 must be unequal. To this end, the two legs 5a and 5b of the probe 5 are made of metal tubes having an outer diameter equal to the diameter of the probe 4 and being otherwise identical to the legs of the probe 4.

As a result of this structure, the two probes present identical surfaces to the uid stream, but the thermal capacities of the two probes are unequal. Y

Also, as illustrated in FIG. 1, the probe 4 is connected through leads 10 and a reference thermocouple junction 8 with a computer9. Likewise, the probe is connected through leads 410 and a reference :thermocouple junction 11 with thefcomputeru9. ,Although for purposes of illustration `the reference thermocouple junctions have been indicated `as external `tofthe computer 19, .asa practical matter ,these elements are built into the computer. As a result of this system,;-theyE.M.F. records ofthetwo probes 4and 5 reflecting theiriinstantaneous temperatures t1 and t2,.are fed intonthe computer x9 forsolvingthe equation tg (the instantaneous temperature inthe'fluid stream) Since the thermal capacity of the air in the tubing of the probe 5 is negligible, and sincethe. area `exposed `to heating is the same for both of the probes 4 and 5, the time constantsof the two will nbe,` proportional to their weights, which can be specified or determined. Thus,

The response of the two thermocouples is givenby the following two equations:

(time constant for thermocouple 5) Substituting this value ,of h in Equation 4, and solving for zg gives or letting AKb=K1/K2i= W1/ W2,

r-Kb

This last equation gives the stream temperature tg as a function of the outputs of the two thermocouples, t1 and t2 and their first time derivatives tl and tz. It should l be observed that the requirements for a probe which will satisfy the conditions of the last above-mentioned equation are that the product hA is the same for each thermocouple and that the product of Wc is different but known for each. The system shown in FIG. l is one means of I satisfying this. `Other `probe designs such as the ones shown in FIGS. 3, 4, 5, `6, `and 7 willbe presentlydescribed.

Here it should` also be observed that an additional modi- 4fication of the invention is aprobe `with two thermo-elements wherein the -heat transfer coefficients (unit thermal conductance) of the'two Vthermo-elements are equal and wherein the `ratio of the exposed area of the first A1 to its thermal capacity Wlcl and ratioof the area` of the second A210 its thermal capacity are unequal, i.e.,

A1/W1C1=A2/W2C2 `The operations involved in the right side of the last -equation are straightforward and canbe performed with `standard electronic components since the probe outputs are voltages. The entiresystem will thus consist of a twothermocouple probe and anelectroniccircuitfto perform the'comput-ation indicated in the above-mentioned last equation, to give anoutput equivalent to the inistantaneous gas temperature tg `as shown schematically in FIG. `1.

` In FIG. 2` a system has been shown wherein the outputsof the probes.4 and 5 are fedrespectively into selfbalancing recording potentiometers p1 and 112,; and wherein the temperatures t1 and t2 of the two thermocouples `4 and 5, or rather the voltagescorresponding thereto, are

readable ,on a millivolt scale 1S-forming a part of the potentiometer. Otherwise, the various elements of this `system are identical with 1 the system of FIG. l and have been designated accordingly.

The `systems illustrated inFIGS. 1 and 2 are merely special `cases `of my invention wherein the surface areas of the twofprobes are made equal, wherein the two probes Aare symmetrically disposed in the fluid stream and wherein the `two `probes are made of the same materials and configuration and therefore have identical heat transfer coeicients h.

.If `theprobes are not symmetrically disposed in the fluidfstream,lbut their orientation is-known, the action of the uid stream on one probe will differ from its action on the other probe merely by a constant which can be calculated. Similarly, ifthelexternal exposed surfaces of thetwo probes are-unequal, but known, the resulting difference in the-E.M.F. ,producedby this discrepancy can be compensated for by a constant. In its more general form, the relationship which must exist between the probes is that-the `,total surface conductance hlAI of one probe be equaltolChgAz, the total surface conductance of the other.

However, the necessity of dealing with the constant C can be obviated by the simple expedient of using two probes having equal surface areas, symmetrically disl posed in the fluid stream and made ofthe same-materials, all as illustrated in-FIGS. l, 2, and 3.

Although the probes illustrated in FIGS. `1, `2y, `and 3 `arein the form of thermocouples, this is not necessary, for, as already suggested, such probes or thermo-elements `,can take the form of resistors,` thermistors, and fluid-filled bulbs, so long aseach pair of probes is symmetrically locatedin the fluid stream, their surface areas being equal, and their thermal` capacities being unequal..

VAs illustrated in FIG. 4,the two probes of my system can take the form of a first resistance wire 42110i one ma- ,teriallmounted in a ceramic supporting block 22 through Ceramic tubes 23 and a second resistance wire 24 of a `different-thermal capacity than the wire 21 but having an identically exposed surface area. In other words, the conditions imposed onthe wires 21 and 24.are identical to the conditions imposedon the thermocouples 4 and 5 as previously described.

As diagrammatically illustrated in FIG. 8, the resist- :ances `2;1;and 24 are;respectively included as one leg of .bridges 31 and ,32. The bridge 31 includes a galvanometer G1 and a variable resistance R1. Likewise, `the bridge 32 `includes a galvanometer G2 and a variablelresistance R2. Connected between the two bridges is'a battery B. The resistance of each of the probe resistances 21 and 24 will vary with the temperature to which it is subjected and this will result in a voltage variation across the bridge associated therewith as reilected by the galvanometer. Each bridge is maintained in balance by adjusting its variable resistance R1 or R2 as the case may be. However, since the response of a human being is not sufficiently fast to permit the bridges to be maintained in balance by manual operation of the variable resistances R1 and R2, resort is had to a servo system wherein a servo motor M1 is made responsive to variations in galvanometer G1, through a suitable electronic circuit and wherein the variable resistance is controlled by the motor M1. Similarly, the bridge 32 is associated with a servo motor M2 for actuating the variable resistance R2 is response to variations in the galvanometer G2. Since well-known equipment can be used for this purpose, a further explanation of its construction and operation is deemed unnecessary.

Variable resistances R1 and R2 reflect the instantaneous temperatures of the probes 21 and 24, and as in the case of the system illustrated in FIG. 1, these parameters can be delivered to a suitable computer for solving for tg in accordance with that last-named equation as above set forth.

For the resistance probe elements 21 and 24 illustrated in FIG. 9, thermistors such as illustrated in FIG. can be used, for broadly speaking, a thermistor is nothing more than a thermally sensitive resistor having a negative temperature coefficient.

As illustrated in FIG. 6, thermistors 41 and 42 are mounted on a ceramic supporting block 43 through the agency of ceramic tubes 44 and leads 45. Preferably, the thermistors 41 and 42 should be made of different materials, but be of the same size and shape, and as in the case of all probes for this purpose, their thermal capacities should be unequal. Y

As illustrated in FIG. 6, the probes take the form of thermally sensitive resistors 51 and 52 encased in protective ceramic tubing 53 extending into or through the ceramic supporting block 54.

Here again for the sake of simplicity the resistors and their protecting tubing should preferably be of identical size and shape and the thermal capacities of the two probes should be unequal.

FIG. 7 is merely illustrative of a modified form of thermocouple probe system wherein two thermocouple elements 61 and 62 are symmetrically embedded in a ceramic supporting block 63 and symmetrically presented to the uid stream 64. Here it should be particularly noted that although the exposed areas of the two probe elements are equal, the thickness of the probe element 62 is greater than the thickness of the probe element 61 so as to satisfy the condition that thermal capacities of the two probes be unequal.

Still another type of probe which can be used as an alternative to thermocouples is a uid-iilled bulb such as is illustrated in FIG. 9. As there shown, a pair of liquidlled bulbs 71 and 72 having identical surface areas but unequal thermal capacities are symmetrically mounted in a ceramic supporting block 70 for symmetrical disposition in a fluid stream. As illustrated in FIG. 9, the bulbs 71 and 72 communicate respectively with sylphons 73 and 74 and which operate in response to variations in the temperatures of the fluid within the bulbs 71 and 72. Associated with the sylphon 73 is a closed circuit including a resistor 75, an ammeter 76, and a variable resistor 77. Shunted around the resistor 77 is a battery 78. The movement ofthe variable resistor operator 79 is made responsive to the movement of the sylphon 73.

Similarly, the sylphon 74 is associated with a closed circuit including a resistor 81, an ammeter 82, and a variable resistor 83 including an operator 84. Shunted around the resistor 83 is a battery 85.

The pressures in the bulbs 71 and 72 are functions of lthe temperatures of the bulbs, VVariations in such pressures is manifested by a corresponding movement of the sylphons associated with the bulbs. And the motion of each sylphon is used to vary the voltage in its associated circuit and in which its ammeter shows a current proportional to the position of its associated sylphon. These currents are then related to the instantaneous temperatures of the bulbs 71 and 72 and can be fed into a cornputer for solving for tg the instantaneous temperature of the iluid or jet stream.

I claim:

l. A temperature sensing system for determining the instantaneous temperature rg of a fluid stream comprising: iirst and second temperature sensing elements syrnmetrcally disposed within said fluid stream for identical action thereby, the total thermal conductances of said sensing elements being equal and their thermal capacities being unequal; and a computer connected with said first and second sensing elements for calculating said instantaneous temperature tg in accordance with the formula:

f1-152K?-1 2 tz: tf1 l-K -t-fz tg=fluid stream temperature varying in general with time T t1=instantaneous temperature indicated by the first sensing element t2=instantaneous temperature indicated by the second sensing element where K1=the thermal capacity of the first sensing element, K2=the thermal capacity of the second sensingrelement.

2. A temperature sensing system for determining the instantaneous temperature tg of a iiuid comprising: first and second thermal elements having unequal thermal capacities, and so disposed in said fluid that the total surface heat transfer conductances of said elements are equal; the output of said thermal elements being connected with a computer for calculating said instantaneous temperature tg in accordance with the formula:

l 1 K ,-1 t 2 wherein tg=iiuid stream temperature varying in general with time T t1=instantaneous temperature indicated by the first sensing element t2=instantaneous temperature indicated by the second sensing element A where lil-:thermal capacity of the rst sensing element, and K2=thermal capacity of the second sensing element.

References Cited in the le of this patent UNITED STATES PATENTS Bristol Feb. 25, 1908 Brown Feb. 21, 1928 De Florez et al. Mar. 19, 1935 De Florez et al Sept. 15, 1936 Dasher Nov. 12, 1940 Barnhart Dec. 30, 1941 10 2,477,835 Smith Aug. 2, 1949 2,660,883 Wyczalek Dec. 1, 1953 2,696,120 Underwood Dec. 7, 1954 OTHER REFERENCES 

2. A TEMPERATURE SENSING SYSTEM FOR DETERMINING THE INSTANTANEOUS TEMPERATURE TG OF A FLUID COMPRISING: FIRST AND SECOND THERMAL ELEMENTS HAVING UNEQUAL THERMAL CAPACITIES, AND SO DISPOSED IN SAID FLUID THAT THE TOTAL SURFACE HEAT TRANSFER CONDUCTANCES OF SAID ELEMENTS ARE EQUAL; THE OUTPUT OF SAID THERMAL ELEMENTS BEING CONNECTED WITH A COMPUTER FOR CALCULATING SAID INSTANTANEOUS TEMPERATURE TG IN ACCORDANCE WITH THE FORMULA: 